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University of Montana

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Glaciology

Abstract

The gradient in hydraulic potential at the ice-bedrock interface beneath the Greenland Ice Sheet (GrIS) dictates the routing and energetics of subglacial water, thereby influencing drainage system characteristics and sliding dynamics. In the ablation zone of the GrIS, a high relief bed and gradients in water pressure in an active subglacial drainage system potentially play a complex role in dictating the hydraulic potential field. Here we present a suite of water pressure measurements collected within thirteen boreholes along a 46 km transect on the western GrIS in order to investigate the sensitivity of the potential gradient field to seasonal evolution and diurnal cycling of basal water pressure. All sites show pressures with similar seasonality, having relatively steady and high values during winter, variable and irregular behavior during spring and fall, and diurnal cycles that can persist for multiple weeks during the peak melt season. Despite much higher variability during the melt season, the median pressure of the summer period is nearly the same as the median pressure of the winter period. Diurnal minimums in water pressure remain above 0.8 of overburden pressure across the transect, except for a single borehole in very near-margin shallow-ice that fell to 0.3 of overburden. We find the mean basal water pressure in western Greenland generally mimics the ice thickness field, but basal drainage processes can superimpose strong gradients over short length scales. Variability of the pressure field due to basal drainage processes can force dynamic changes in the region’s hydraulic potential gradient.

High time resolution GPS surface velocity records were collected at 4 sites along the transect during 2011-2015, providing at least two consecutive melt season velocity records at each site, and over-winter records at two sites. Here, I describe methods of GPS station deployment and operation, and data processing and post-processing procedures. Velocity at all sites shows seasonal phases including: a multi-day increasing ramp in the spring from winter background velocities; a period of regular diurnal cycles during mid-summer; and a period of more irregular and variable behavior in the fall, typically characterized by episodic single- or multi-day speedup events. At all sites, linear regression shows decreasing trends in velocity over the melt season. Winter velocity at two sites has magnitude of ~100-120 m/yr, and persists for 9-10 months of the year. In both over- winter records, velocity slowly increases by 10-20 m/yr over the course of the winter.